Late blight has showed up on tomato plants from New York to Wisconsin, lowering crop yields and profits for both organic and conventional farmers. While growers struggle to ward off the devastating disease, there are important signs for consumers and home gardeners to look out for in order to prevent late blight from spreading further.

According to the Wisconsin Ag Connection, late blight results from a pathogen called Phytophthora infestans and "is spread by spores carried by rain, wind, people, machinery and wildlife." More than a century ago, late blight contributed to the Irish potato famine; the disease also affects eggplant and certain flowering plants.

ScienceDaily explains that the early visible symptoms of late blight are often brownish lesions on plant stems. The spots grow quickly, and rainy weather typically leads to development of "white fungal growth" and the collapse of the stem. Weekly examinations of plants, and regular spraying with chlorothalonil-containing fungicide products, are recommended measures. Infected plants should also be destroyed.

The late blight of 2009 has been especially devastating for two reasons, according to Cornell University's Vegetable MD Online. First, the disease occurred very early in the growing season. Secondly, infected plants were widely distributed to regional retailers.

For organic farmers, late blight is even more difficult to respond to. Harsh chemicals are not an option, and the copper treatments that are typically used have no effect once a plant has been infected, according to eXtension.

This summer, many organic farmers "were forced to make a brutal choice: spray their tomato plants with fungicides, and lose organic certification, or watch the crop disappear," Dan Barber, an op-ed contributor for The New York Times, wrote. Barber, the chef and co-owner of Blue Hill and Blue Hill at Stone Barns, says "the explosion of home gardeners" has helped late blight spread. He calls for a greater sense of community among Americans who grow food, whether for personal or commercial use, and emphasizes the importance of agricultural education.

Interestingly, Barber also contends that his and other sustainability advocates' suspicions of conventional farming methods, and their reluctance to accept "science, when it's applied to agriculture," could be thwarting progress of disease-resistant plants.

Cornell University's New York State Integrated Pest Management (NYSIPM) provides detailed information about late blight, including how it spreads, how to spot early symptoms and how to prevent it. According to NYSIPM, the oomycete pathogen that causes late blight "survives from one season to the next in infected potato tubers," requiring growers to destroy all infected plants.

Encyclopedia Britannica offers comprehensive background information on the Irish potato famine, caused by late blight. Nearly half of Ireland relied "almost exclusively on the potato for their diet" in the early 1840s. When late blight spread to Ireland from North America in 1845, a year that also featured "unusually cool, moist weather, in which the blight thrived," it resulted in crop failures. The blight grew worse each year through the late 1840s, ultimately cutting Ireland's population from about 8.4 million in 1844 to 6.6 million in 1851.

The University of California Museum of Paleontology's "Understanding Evolution" explains one of the causes of the Irish potato famine: a lack of genetic variation. Ireland's complete dependence on the "lumper" variety of potato made it easier for the blight to spread.